1. Field of the Invention
This invention relates, in general, to photocopying and, more specifically, to copiers and printers which use digital data to produce the hardcopy output image.
2. Description of the Prior Art
Copiers, printers, and like devices can use various types of input devices to provide the image data necessary to construct the output image, or hard-copy. Printers and electronic copiers, where the original document is scanned to obtain digital data, are capable of electronically processing the image data. Frequently, such processing is used to enhance the quality of the hard-copy output, reduce the memory requirements for economical storage of the image data in the device, and/or lower the data transmission time between device systems. Processing to reduce memory requirements is especially helpful when gray-levels are involved and when the devices are capable of rendering the output in color as well as monochrome.
Other image processing techniques are used in addition to compression. Halftoning is used to condition continuous tone image data for representation by a limited gray-level output rendering system, or print engine. Because such data is in digital form, digital halftone processing may be used to create the halftone screen which develops the halftone cells. However, both halftoning and compressing tend to cause a reduction in the sharpness or resolution of the image content. This occurs primarily because the halftone cell includes several pixels which are added or totaled together to determine the lightness or density level for the overall cell. The resolution provided by the individual pixels is lost because of the use of the pixel to establish the total density of the halftone cell.
Various prior art systems have been proposed to increase the effective resolution in halftoned cells. One such system is described in U.S. Pat. No. 4,246,614, issued on Jan. 20, 1981. The system described in this patent shifts the "black" center of the halftone cell to align the center-printed pixel with the corresponding "black" center in the video image signal. See column 3, lines 3-14. Other systems have been known in the prior art wherein dispersed dot or error diffusion have been used to improve the halftone resolution. While some of these offer certain advantages, they are computationally intensive and some offer no reduction in the memory space requirements.
Therefore, it is desirable, and an object of this invention, to provide gray-level, halftone, image processing which reduces memory requirements and preserves the resolution of the output image.